1. Reference to Related Applications
Reference is made to another United States patent application filed concurrently with the present application, both being assigned to SRI International and having as one of their inventors, James F. Wolfe. Said other application is entitled: "Liquid Crystalline Poly(2,6-benzothiazole) Compositions, Process, and Products" and is herein incorporated by reference.
2. Background Art
In general, the class of aromatic heterocyclic extended chain polymers are well known for their outstanding thermal, physical, and chemical properties. Unfortunately, these polymers are essentially non-melting and have proven very difficult to economically process into articles. In order to fashion such polymers into desired articles of commerce, for example fibers, films, fibrids, and the like, it is necessary that they be in solution or dope form. Although such polymers can be dissolved in various acidic solvents, such as sulfuric acid, methanesulfonic acid, chlorosulfonic acid, polyphosphoric acid, and the like, difficulty is often experienced in preparing and using the polymer-acid compositions or dopes because of poor polymer-acid solubility.
Normally, a precipitated or dried particulate form of the polymer is dissolved in a strong acidic solvent by mixing the (isolated) polymer particles at elevated temperatures and/or under high pressures for a period from several hours to several days. If the polymer is insoluble in the particular solvent, other solvents or various solvent mixtures are employed. Usually, heating and cooling cycles are applied and repeated to obtain a useful dope.
The resulting dopes often contain undissolved polymer and must be filtered before further processing into articles.
Although spinning dopes of polybenzobisoxazole, polybenzimidazole and polybenzobisthiazole in sulfuric acid and/or methanesulfonic acid and/or chlorosulfonic acid with polymer concentrations above about 10 percent are known in the art, the intrinsic viscosity of these polymers is for the most part below 5dL/g and oftentimes less than 3dL/g. The cohesive strength of such dopes is inherently weak and economically less desirable for use in dry-jet wet spinning. In the case of polybenzobisoxazole, numerous attempts of dry-jet wet spinning an approximately 10% polybenzobisoxazole/methane sulfonic acid-dope into fibers were not successful (E. W. Choe, et al., in Macromolecules 1981, 14, pp. 920-924).
In the case of polybenzimidazole, prior art dopes of this polymer lack adequate strength to maintain filament integrity while dropping through the air-gap. In order to overcome this problem U.S. Pat. No. 4,263,245 teaches dissolving a high concentration (up to 30%) of this polymer into suitable solvents such as concentrated sulfuric acid. At such high polymer concentrations lithium chloride is required to prevent the polybenzimidazole from phasing out of solution.
In the case of polybenzobisthiazole, U.S. Pat. No. 4,225,700 teaches the formation of a liquid crystalline composition of this polymer at concentrations near 10% in methane sulfonic acid and chlorosulfonic acid and at about 6% in polyphosphoric acid. Concentrations of polybenzobisthiazole in polyphosphoric acid above about 10% by weight are difficult, if indeed possible to achieve. One difficulty encountered is that the solution of the 2,5-diamino-1,4-benzenedithiol monomer in polyphosphoric acid with the P.sub.2 O.sub.5 content described in U.S. Pat. No. 4,225,700 is very viscous and dehydrohalogenation is difficult. Also considerable foaming results. Although solutions of precipitated polymer in solvents such as methane sulfonic acid and chlorosulfonic acid can be prepared, high concentrations of polymer are difficult or impossible to achieve. S. R. Allen, et al., in Macromolecules 1981, 14, pp. 1135-1139 describes attempts at spinning polybenzobisthiazole directly from the polymerization medium (polyphosphoric acid) containing 5-6% polymer.
Insofar as polybenzobisthiazole is concerned it is possible to obtain compositions near to 10% of the polymer in polyphosphoric acid with intrinsic viscosity equal to 26dL/g (J. F. Wolfe, et al., Macromolecules 1981, 14, pp. 915-920). Attempts to increase the intrinsic viscosity of the polymer can only be achieved at a major sacrifice (decrease) in polymer concentration. Liquid crystalline compositions of 10% polybenzobisthiazole in polyphosphoric acid are heretofore unknown in the art. Liquid crystalline compositions of polybenzobisthiazole having intrinsic viscosities greater than about 30.3dL/g in polyphosphoric acid are heretofore unknown in the art.
In practical terms this means that such polymerpolyphosphoric acid compositions are severely limited in their potential usefulness for the production of highly ordered high molecular weight polymeric articles.
In general, liquid crystalline extended chain polymer compositions (with the exception of polybenzobisthiazole as mentioned above) in polyphosphoric acid are heretofore unknown in the art; and moreover, liquid crystalline extended chain copolymer and block polymer compositions are heretofore unknown in the art.